1. Field of the Invention
This invention relates to recording and reproducing systems for heart sounds and, more particularly, to such systems having the capability of recording heart sounds from a stethoscope and playing them back through the stethoscope at a reduced signal rate but at the natural pitch.
2. Description of the Prior Art
Physicians and others have been accustomed to listening to heart sounds for centuries. The invention of the stethoscope is attributed to R. T. H. Laennec who, in 1816, used a wooden cylinder to transmit heart sounds to the ear. Acoustic stethoscopes are used to investigate the condition of the respiratory, circulatory and digestive systems. Most sounds of interest to a doctor, especially heart sounds, have frequencies in the range from 60 to 600 Hz, but some mitral diastolic murmurs (irregular sounds heard over the heart during expansion of the heart and indicating an abnormality in the mitral valve) have frequencies below 60 Hz; a few sounds, such as crepitations (crackling sounds heard over the chest in some diseases of the lungs), have frequencies up to about 1400 Hz. Acoustic stethoscopes do not amplify sound; they merely convey it to the ear in as efficient a manner as possible.
Phonocardiographs are devices for monitoring heart sounds electronically. A phonocardiograph gathers no more information than a simple acoustic stethoscope, but it displays the sound in visual form on a cathode ray tube or pen recorder, sometimes making diagnosis simpler. Electrocardiograms are also used to diagnose heart valve defects. These, however, are recordings or displays of voltage patterns, rather than heart sounds, which are generated by the heart muscles. The medical field has long been aware of the use of digital computers in the analysis of electrocardiac signals and the automatic measurements of various parameters in the heart wave as developed in an electrocardiogram. Some early studies in this area, initiated by Dr. Cesar A. Caceres, then of the United States Public Health Service, have been reported, among others, in "Pattern Recognition in the Clinical Electrocardiogram" by Caceres et al, IRE TRANSACTIONS ON BIO-MEDICAL ELECTRONICS, January 1962.
Despite the capability of the prior art to analyze electrical signals from the heart, such developments do not deal with analysis of heart sounds. They may provide correlative information but they require equipment which is vastly more complex than the simple stethoscope with which every doctor is familiar and is generally equipped. There are examples in the prior art of attempts to make heart and other body sounds, particularly of low and almost inaudible frequencies, more readily audible by conversion into a higher frequency range. See for example Gregg U.S. Pat. No. 3,348,535, Massie U.S. Pat. No. 3,601,120 and the British U.S. Pat. No. 1,008,027. These, however, unavoidably alter the character of the sounds thus converted and, accordingly, do not deal with the objective of processing heart sounds to provide a result which is unaltered as to pitch, and therefore will be familiar to the practitioner, but stretched out in time so that the practitioner can more readily distinguish and analyze the individual portions of the cyclic heartbeat sound. Furthermore, these patented systems do not deal with the desirable feature of permitting the practitioner to repeat, stretched out in time, a heart cycle or a few cycles which he has just heard and would like to hear again.
Stethoscope recording systems are well known in the prior art. For examples of such, see U.S. Pat. Nos. 3,052,756 of Seven et al, 3,188,645 of Trumpy et al, and 3,846,585 of Slosberg et al. However, these only provide the capability of repeatedly reproducing the recorded heart sounds for further study, with the advantage of amplification and tonal modification, if desired.
So-called "electronic stethoscopes" have been disclosed, for example, in U.S. Pat. Nos. 3,160,708 of Andries et al, 3,132,208 of Dymski et al, 3,247,324 of Cefaly et al, and 4,170,717 of Walshe. Typically, however, these disclosures merely deal with the capability of amplifying the sounds picked up by the stethoscope and possibly processing the heartbeat signals to remove undesired sounds so that other sounds can be amplified, studied and/or recorded without interference from the undesired sounds.
Systems are known for the time compression and expansion of audio signals. Those of which I am aware generally depend upon very sophisticated and complex equipment, as well as techniques and concepts, and are designed for dealing with audio signals other than heart and body sounds. One such system of Kitamura, disclosed in U.S. Pat. No. 3,975,763, operates with signals having been reproduced with a frequency spectrum scale differing from that at the time of recording. The Kitamura et al U.S. Pat. No. 4,020,291 utilizes a filter for filtering the fundamental frequency component of an input signal reproduced at a speed differing from that at the time of recording to produce a time compressed or expanded signal synchronized with the pitch period of the fundamental frequency component of the input signal. Boothroyd in U.S. Pat. No. 3,520,996 discloses apparatus for playing back an original recording at a slower speed with reduced pitch. The signal is then processed by delay line sampling in a reverse direction to increase the pitch, and thereafter the samples are inserted periodically into the original record to extend the playback time. Schiffman in U.S. Pat. No. 3,828,361 discloses storage of speech signals in analog shift registers with control of the shift rate in accordance with reproducer input speed so that normal frequency sounds are developed, regardless of input pitch level or transducer speed.
Despite these examples of prior art developments, the problem still remains of adequately dealing with the transitory nature of heart sounds which the physician hears through his stethoscope and processing them so that he is able to hear the sounds in the same manner and at normal pitch but stretched out in time. Heart sounds are audible representations of certain physical events, notably closure of certain valves and alterations in fluid flow. The sounds generated by these events vary in duration, pitch, timbre, volume and sequence. It is by interpretation of these sounds that the sophisticated ear can infer the presence of disease and often the nature of such disease. These various sounds occur in rapid succession and may actually overlap one another. Rapid heart rates shorten the duration of each heart cycle, rendering interpretation of these sounds more difficult. Simple amplification may assist in low volume problems or problems of high ambient noise, and selective frequency amplification may aid in interpretation. However the principal problem is the compression of all of the pertinent information into a short time span. The present invention is designed to stretch the duration of the cycle of heart sounds without altering the pitch, sequence, character and relative duration of these sounds, thus simplifying such interpretation by the ear of the examiner. In using the stethoscope, the practitioner would like to be able to select from time to time a few heartbeats, say five in sequence, and play them back for review. Desirably, these should be replayed stretched out in time, say five to ten times the real time interval, and without changing the pitch or other audible characteristics of the sounds.